Axial magnetic bearing apparatus

ABSTRACT

In axial magnetic bearing apparatus in which a rotary disc made of a magnetic material is fixedly attached to a rotating shaft, and electromagnets are disposed on opposite sides of the rotary disc so as to have suitable very small distances therefrom respectively, so as to bear the rotating shaft axially in a non-contact state, a deep groove for forming an air layer having large magneto-resistance is provided all over the outer circumference of the rotary disc. Thus, formation of a magnetic circuit not contributing to position control is relieved, and the weight of the disc is reduced so that the position control performance of the axial magnetic bearing and the natural frequency of a rotor are improved.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 10/868,946 filed Jun. 17,2004, which is a divisional of application Ser. No. 10/089,547 filedApr. 1, 2002 (issued as U.S. Pat. No. 6,781,269 on Aug. 24, 2004), whichis a National Stage Entry of PCT Application No. PCT/JP99/05362 filedSep. 30, 1999 and is considered part of the disclosure of theaccompanying divisional application and are hereby incorporated byreference.

TECHNICAL FIELD

The present invention relates to axial magnetic bearing apparatus asfollows. That is, in the axial magnetic bearing apparatus, a rotary discmade of a magnetic material is fixedly attached to a rotating shaft ofrotating apparatus such as an electric generator, an electric motor, orthe like. Electromagnetic stators each having an electromagnetic coilfor generating magnetomotive force are fixed to casings respectively soas to be located with very small distances from the rotary disc. Adisplacement sensor for measuring axial displacement of the rotatingshaft is provided. Magnetic attraction force is made to act between therotary disc and the electromagnetic stators in accordance with an outputsignal from the displacement sensor, so as to bear the rotating shaft ata target position distant from the electromagnetic stators and innon-contact therewith.

BACKGROUND ART

FIG. 8 shows a background-art general example of so-called axialmagnetic bearing apparatus for bearing a rotating shaft of rotatingapparatus such as an electric generator, an electric motor, or the like,in a thrust direction by use of magnetism. In the drawing, the referencenumeral 1 represents a rotating shaft, to which a rotary disc 2 made ofa magnetic material is fixedly attached. The rotary disc 2 usually hassleeves 6 formed on opposite sides of the rotary disc 2 so as to makethe fixation of the rotary disc 2 to the rotating shaft 1 firm. Thereference numeral 10 represents each of ring-like electromagnetic coilsformed by winding coated copper wire around the rotating shaft 1 with arequired and adequate number of turns. Each of the electromagnetic coils10 is incorporated in an electromagnetic stator 7 having an insidemagnetic pole tooth 11 and an outside magnetic pole tooth 12. Thereference numeral 8 represents each of ring-like housings which forms amagnetic circuit portion for corresponding one of the electromagneticstators 7. The electromagnetic coils 10 are received in coil slots 9formed symmetrically with respect to the rotation axis of the ring-likehousings 8. The electromagnetic stators 7 are paired and disposed inopposition to each other with respect to a collar 22 so as to havesuitable very small distances from the rotary disc 2 respectively onopposite sides of the rotary disc 2. Thus, the electromagnetic stators 7are attached to the casings 23.

This axial magnetic bearing apparatus is controlled as follows. That is,axial displacement of the rotating shaft 1 is measured by a not-showndisplacement sensor. On the basis of an output signal from thisdisplacement sensor, electric currents to the electromagnetic coils 10are adjusted to suitably vary magnetic attraction force acting betweenthe rotating disc 2 and the inside magnetic pole teeth 11 of theelectromagnetic stators 7 and between the rotating disc 2 and theoutside magnetic pole teeth 12 of the electromagnetic stators 7. Thus,the rotating shaft 1 is borne at a target position distant from theelectromagnetic stators 7 and in non-contact therewith.

However, in the structure of the above-mentioned axial magnetic bearingapparatus generally used in the background art, for example, magneticcircuits as shown in FIG. 9 are formed among the two electromagneticstators 7 a and 7 b and the rotary disc 2 by selecting the polarities ofthe electric currents flowing into the electromagnetic coils 10. At thistime, there are two magnetic circuits 13 formed between the respectiveelectromagnetic stators 7 and the rotary disc 2, and a magnetic circuit14 formed between the two electromagnetic stators 7 a and 7 b opposed toeach other with respect to the rotary disc 2. Here, the magneticcircuits 13 are magnetic circuits which contribute to magneticattraction force required for the position control of the axial magneticbearing, but the magnetic circuit 14 is a magnetic circuit which doesnot contribute to the magnetic attraction force at all.

As a result, the magnetic attraction force generated by each of theelectromagnetic stators 7 decreases so that the support stiffness of theaxial magnetic bearing apparatus decreases.

Incidentally, the reason why the support stiffness decreases due to thegeneration of the magnetic circuit 14 is, for example, disclosed inJapanese Patent Laid-Open No. 122896/1993. Therefore, the description ofthe reason is omitted here.

Thus, an invention for improving this defect is, for example, disclosedin Japanese Patent Laid-Open No. 122896/1993. FIG. 11 shows thisbackground-art improved axial magnetic bearing apparatus. In thedrawing, one rotary disc piece 3 made of a magnetic material has anL-shaped sectional structure with a sleeve 6. A pair of such rotary discpieces 3 are opposed to each other on their contra-sleeve sides, and adisc 5 of non-magnetic material is sandwiched like a layer between therotary disc pieces 3. Thus, one rotary disc 2 is formed. Electromagneticstators 7 are disposed respectively with suitable very small distancesfrom the rotary disc 2 on opposite sides of this rotary disc 2 so as tobe opposed to each other with respect to a collar 22. Thus, theelectromagnetic stators 7 are attached to casings 23.

Accordingly, a magnetic circuit 14 which is formed through the twoelectromagnetic stators 7 opposed to each other with respect to therotary disc 2 and which does not contribute to magnetic attraction forceis eliminated. On the other hand, independent magnetic circuits 13 areformed between the respective electromagnetic stators 7 and the rotarydisc 2. Thus, the performance of position control of the axial magneticbearing apparatus is improved.

Now, generally, axial magnetic bearing apparatus is often used as asupport mechanism for a high-speed rotating body. It is difficult torealize such a support mechanism by a mechanical contact type bearing.In the high-speed rotating body, the natural frequency of thefirst-order bending mode of a rotor is important when the dimensions andshape of the rotor are designed. It is requested to design the rotor tohave a natural frequency as high as possible. To this end, the strengthand mass of the rotary disc 2 and the fixation stiffness between therotating shaft 1 and the rotary disc 2 often become critical in theaxial magnetic bearing apparatus which generally has a maximum outerdiameter in the rotor shape. It is therefore necessary to pay closeattention to the design of the rotor shape, particularly the design ofthe shape of the rotary disc 2 of the axial magnetic bearing apparatus.

Generally, when a rotor makes a rotary motion, centrifugal force F [N]as shown in the following expression acts on the rotating body, and themagnitude thereof is in proportion to the mass and the outer diameter ofthe rotating body.F=mrω²

Provided that m designates the mass [Kg] of the rotating body, rdesignates the outer radius [m] of the rotating body, and ω designatesthe rotation angular velocity [rad/sec].

The rotary disc 2 of the above-mentioned improved axial magnetic bearingapparatus (FIG. 11) in the background art has two rotary disc pieces 3.Each of the rotary disc pieces 3 has an L-shaped sectional structurewith a sleeve 6. The two rotary disc pieces 3 are opposed to each otheron their contra-sleeve sides, and a non-magnetic disc 5 is sandwichedbetween the rotary disc pieces 3 so as to form one rotary disc. Thus,the two rotary disc pieces 3 and the non-magnetic disc 5 are independentof one another, and not locked to one another. As shown in FIG. 12, atthe time of high speed rotation, centrifugal force 24 acts on the rotarydisc 2 and the non-magnetic disc 5. Thus, the rotary disc 2 has amaximum outer diameter at rotation-axis-direction positions of angularportions 4 of the rotary disc pieces 3. As a result, larger centrifugalforce acts on the rotary disc 2 at the positions than in any portion ofthe sleeves 6. Thus, maximum stress is generated in the angular portions4 due to the centrifugal force at the time of high speed rotation. A gap25 between the rotating shaft 1 and the rotary disc 2 becomes maximal atthe positions of the angular portions 4. Thus, there arises a problemthat the fixation between the rotating shaft 1 and the rotary disc 2 isretained only at a part of the sleeves 6. On the other hand, the samething can be applied to the non-magnetic disc 5. Since the non-magneticdisc 5 has a maximum outer diameter, there is produced a gap between therotating shaft 1 and the inner diameter of the non-magnetic disc 5 sothat the fixation cannot be retained perfectly.

Further, the natural frequency of the first-order bending mode of arotating body is generally in proportion to the square root of thereciprocal of the rotor mass. It is therefore advantageous that when ahigh-speed rotation rotor is designed, the number of fixation partsresulting in additional mass to thereby cause the decrease in the rotorstiffness is reduced to the utmost so that the weight of the rotor isreduced. However, the rotary disc 2 of the above-mentionedbackground-art improved axial magnetic bearing apparatus has a structurein which the rotary disc 2 is divided into two pieces, and thenon-magnetic disc 5 is added between the two rotary disc pieces 3. Thus,there has been also a problem that such a structure is disadvantageousbecause the number of parts is increased, and the rotor mass is alsoincreased.

In addition, another embodiment of the above-mentioned background-artimproved axial magnetic bearing apparatus has a structure in which anair layer in place of the non-magnetic disc 5 is sandwiched between thetwo rotary disc pieces 3. Also in this case, similarly to theabove-mentioned embodiment, the two rotary disc pieces 3 are independentof each other and not locked to each other. Thus, the fixation of theangular portions 4 is spoiled due to centrifugal force at the time ofhigh speed rotation. As a result, a maximum gap is generated so that thefixation between the rotating shaft 1 and the rotary disc 2 is retainedonly at a part of the sleeves 6.

That is, there has been a problem that it is difficult to apply theabove-mentioned background-art improved axial magnetic bearing apparatusto a mechanism for supporting a high-speed rotating body.

In addition, typically, an iron-based magnetic material is often usedfor the casings 23 and the collar 22 to which the electromagneticstators 7 are attached, from the point of view of the manufacturingcost, the cutting workability, and so on. Thus, in the structure of thebackground-art axial magnetic bearing apparatus (FIG. 8), a magneticcircuit 15 as shown in FIG. 10 may be formed. Incidentally, though notshown, also in the improved thrust bearing apparatus shown in FIG. 11, asimilar magnetic circuit 15 is formed when the casings 23 and the collar22 are formed out of an iron-based magnetic material. At this time,there are two magnetic circuits 13 formed between the respectiveelectromagnetic stators 7 and the rotary disc 2, and a magnetic circuit15 formed among the two electromagnetic stators 7 opposed to each otherwith respect to the rotary disc 2, and the collar 22 or the casings 23.Here, the magnetic circuits 13 are magnetic circuits which contribute tomagnetic attraction force required for the position control of the axialmagnetic bearing, while the magnetic circuit 15 is a magnetic circuitwhich does not contribute to the magnetic attraction force at all.

Accordingly, also in this case, the magnetic attraction force generatedby each of the electromagnetic stators 7 decreases so that the supportstiffness of the axial magnetic bearing decreases.

The present invention was devised to solve the foregoing problems. Anobject of the invention is to provide axial magnetic bearing apparatushaving a structure in which formation of any magnetic circuit notcontributing to position control of the axial magnetic bearing isrelieved or prevented to provide high efficiency and superiorcontrollability, and having a structure in which the stiffness of arotor is not lowered even at the time of high speed rotation.

DISCLOSURE OF THE INVENTION

In order to solve the above-mentioned problems, the present inventionprovides axial magnetic bearing apparatus in which a rotary disc made ofa magnetic material is fixedly attached to a rotating shaft, while apair of electromagnetic stators in each of which a ring-likeelectromagnetic coil for generating magnetomotive force is inserted intoa coil slot are fixed to casings respectively so as to be located onopposite sides of the rotary disc with suitable very small distances,and on the basis of an output signal of a displacement sensor formeasuring axial displacement of the rotating shaft, magnetic attractionforce is made to act between the rotary disc and each of theelectromagnetic stators so as to bear the rotating shaft in a targetposition distant from the electromagnetic stators and in non-contacttherewith, wherein a deep groove for forming an air layer having largemagneto-resistance is provided in a vicinity of an axial center of therotary disc so as to extend from an outer circumferential portion of therotary disc toward the rotating shaft, and a bottom portion of the deepgroove is located to be closer to the above-mentioned rotating shaftthan inside magnetic pole teeth of the above-mentioned electromagneticstators. In addition, according to the present invention, theabove-mentioned deep groove is formed all over the outer circumferenceof the above-mentioned rotary disc. In addition, according to thepresent invention, fan-shaped through holes for forming an air layerhaving large magneto-resistance are provided in the vicinity of an axialcenter of the above-mentioned rotary disc so as to extend from an outercircumferential portion of the above-mentioned rotary disc to theabove-mentioned rotating shaft, while walls of rotary disc pieceslocated on axially opposite sides of this through holes are formed assolid walls having no through hole axially.

Further, according to the present invention, the deep groove is formedall over an outer circumference of the rotary disc, while an innerdiameter of the deep groove for forming an air layer having largemagneto-resistance is provided in a vicinity of an axial center of therotary disc so as to extend from an outer circumferential portion of therotary disc toward the rotating shaft, and a bottom portion of the deepgroove is located to be closer to the above-mentioned rotating shaftthan inside magnetic pole teeth of the above-mentioned electromagneticstators. In addition, according to the present invention, theabove-mentioned deep groove is formed all over the outer circumferenceof the above-mentioned rotary disc. In addition, according to thepresent invention, fan-shaped through holes for forming an air layerhaving large magneto-resistance are provided in the vicinity of an axialcenter of the above-mentioned rotary disc so as to extend from an outercircumferential portion of the above-mentioned rotary disc to theabove-mentioned rotary shaft, while walls of rotary disc pieces locatedon axially opposite sides of this through holes are formed as solidwalls having no through hole axially.

Then, according to these configurations, magnetic interference betweentwo magnetic circuits formed by the respective electromagnetic statorsis relieved. As a result, it is possible to relieve the formation of anabnormal magnetic circuit extending from one electromagnetic stator tothe other electromagnetic stator through the rotary disc and extendingfrom the other electromagnetic stator to the one electromagnetic statorthrough the rotor disc. Thus, electric energy supplied to theelectromagnetic coils is effectively utilized to control the position ofthe rotating shaft so that it is possible to improve the controlperformance. In addition, the rotary disc is integrally coupled in theportion where no deep groove or no through hole is provided. Thus, evenif maximum stress is generated due to centrifugal force at the time ofhigh speed rotation in the rotation-axis-direction position of the innerdiameter of the rotary disc having a maximum outer diameter, the stressis relieved so that the fixation between the rotating shaft and therotary disc is retained surely.

Further, the mass of the rotary disc can be reduced by the formation ofthe deep groove or the through holes, and the number of fixation partscan be reduced. As a result, the natural frequency of the rotor can beincreased.

Further, according to the present invention, a distance between asurface of the rotary disc located in a position not opposed to any oneof an inside magnetic pole tooth and an outside magnetic pole tooth ofcorresponding one of the electromagnetic stators and a surface of thecorresponding electromagnetic stator opposed to the surface of therotary disc is formed to be larger than a distance between a surface ofthe rotary disc located in a position opposed to each of the insidemagnetic pole tooth and the outside magnetic pole tooth of thecorresponding electromagnetic stator and a surface of the correspondingelectromagnetic stator opposed to the surface of the rotary disc.

As a result, magnetic flux and leakage flux entering the inside of therotary disc through the surface of rotary disc not opposed to any one ofthe inside magnetic pole teeth and the outside magnetic teeth of theelectromagnetic stators are relieved so that the magnetic flux densityin the inside magnetic pole tooth and the outside magnetic pole tooth ofeach of the electromagnetic stators can be increased. Accordingly, theelectric energy supplied to the electromagnetic coils can be effectivelyutilized to control the position of the rotating shaft.

Further, according to the present invention, slits large enough toincrease radial magneto-resistance are provided at several places inouter circumferential portions of the electromagnetic stators.Accordingly, magnetic interference between two magnetic circuits formedby the respective electromagnetic stators is relieved. As a result, itis possible to relieve the formation of an abnormal magnetic circuitextending from one electromagnetic stator to the other electromagneticstator through the casings or the collar and extending from the otherelectromagnetic stator to the one electromagnetic stator through therotor disc. Thus, electric energy supplied to the electromagnetic coilsis effectively utilized to control the position of the rotating shaft sothat it is possible to improve the control performance.

Further, according to the present invention, an outer circumferentialgroove for forming an air layer having large magneto-resistance isprovided in a portion of each of the outside magnetic pole teeth of theelectromagnetic stators not opposed to the rotary disc so as to extendaxially from a side where the rotary disc is located. Accordingly,magnetic interference between two magnetic circuits formed by therespective electromagnetic stators is relieved, and the magnetic fluxdensity in the inside magnetic pole tooth and the outside magnetic poletooth of each of the electromagnetic stators can be increased. As aresult, it is possible to relieve the formation of an abnormal magneticcircuit extending from one electromagnetic stator to the otherelectromagnetic stator through the casings or the collar and extendingfrom the other electromagnetic stator to the one electromagnetic statorthrough the rotor disc. Thus, electric energy supplied to theelectromagnetic coils is effectively utilized to control the position ofthe rotating shaft so that it is possible to improve the controlperformance.

Further, according to the present invention, an outer diameter of eachof the electromagnetic stators is formed to have substantially as largeas an outer diameter of the rotary disc, and a ring made of anon-magnetic material having a radial thickness enough to form a layerwith large magneto-resistance is interposed between an outercircumferential portion of each of the electromagnetic stators and aninner circumferential portion of corresponding one of the casings towhich the electromagnetic stator is attached. Accordingly, two magneticcircuits formed by the respective electromagnetic stators are insulatedfrom each other magnetically perfectly, and the magnetic flux density inthe inside magnetic pole tooth and the outside magnetic pole tooth ofeach of the electromagnetic stators can be increased. As a result, it ispossible to more surely prevent the formation of an abnormal magneticcircuit extending from one electromagnetic stator to the otherelectromagnetic stator through the casings or the collar and extendingfrom the other electromagnetic stator to the one electromagnetic statorthrough the rotor disc. Thus, electric energy supplied to theelectromagnetic coils is effectively utilized to control the position ofthe rotating shaft so that it is possible to improve the controlperformance.

Further, according to the present invention, a collar made of anon-magnetic material for relatively positioning where the pair ofelectromagnetic stators are attached is provided between the pair ofelectromagnetic stators. Accordingly, two magnetic circuits formed bythe respective electromagnetic stators are insulated from each othermagnetically. As a result, it is possible to more surely prevent theformation of an abnormal magnetic circuit extending from oneelectromagnetic stator to the other electromagnetic stator through thecollar and extending from the other electromagnetic stator to the oneelectromagnetic stator through the rotor disc. Thus, electric energysupplied to the electromagnetic coils is effectively utilized to controlthe position of the rotating shaft so that it is possible to improve thecontrol performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing Mode 1 for carrying out axialmagnetic bearing apparatus according to the present invention.

FIG. 2A is a sectional view showing Mode 2 for carrying out the axialmagnetic bearing apparatus according to the present invention, and FIG.2B is a sectional view taken on line X-X in FIG. 2A.

FIG. 3 is a sectional view showing Mode 3 for carrying out the axialmagnetic bearing apparatus according to the present invention.

FIG. 4A is a sectional view showing Mode 4 for carrying out the axialmagnetic bearing apparatus according to the present invention, and FIG.4B is a sectional view taken on line X-X in FIG. 4A.

FIG. 5 is a sectional view showing Mode 5 for carrying out the axialmagnetic bearing apparatus according to the present invention.

FIG. 6A is a sectional view showing Mode 6 for carrying out the axialmagnetic bearing apparatus according to the present invention, and FIG.6B is a sectional view taken on line X-X in FIG. 6A.

FIG. 7 is a sectional view showing Mode 7 for carrying out the axialmagnetic bearing apparatus according to the present invention.

FIG. 8 is a sectional view showing background-art axial magnetic bearingapparatus.

FIG. 9 is a sectional view for explaining a defect of the background-artaxial magnetic bearing apparatus.

FIG. 10 is a sectional view for explaining another defect of thebackground-art axial magnetic bearing apparatus.

FIG. 11 is a sectional view showing another background-art axialmagnetic bearing apparatus.

FIG. 12 is a diagram showing the deformation of the structure of theother background-art axial magnetic bearing apparatus at the time ofhigh speed rotation.

BEST MODE FOR CARRYING OUT THE INVENTION

Modes for carrying out the present invention will be described below indetail with reference to FIGS. 1 to 7. Mode 1.

FIG. 1 is a sectional view showing Mode 1 for carrying out axialmagnetic bearing apparatus according to the present invention. Ring-likecoil slots 9 are formed symmetrically with respect to the rotation axesof ring-like housings 8 respectively formed out of a magnetic materialwhich is superior in magnetic property. Ring-like electromagnetic coils10 for generating magnetomotive force are inserted into the respectivecoil slots 9 so as to form electromagnetic stators 7 respectively. Arotary disc 2 made of a magnetic material is fixedly attached to arotating shaft 1. The above-mentioned electromagnetic stators 7 aredisposed respectively on opposite sides of the rotary disc 2 withsuitable very small distances from the rotary disc 2. Sleeves 6 to befixedly attached to the rotating shaft 1 are provided on opposite sidesof the rotary disc 2. A deep groove 16 is formed in the vicinity of theaxial center of the rotary disc 2 so as to cover the outer circumferenceof the rotary disc 2 and to extend through the positions of insidemagnetic pole teeth 11 of the electromagnetic stators 7 up to thevicinities of positions short of the inner circumferential positions ofthe sleeves 6. Incidentally, an air layer formed by this deep groove 16is formed with a certain suitable width in the axial direction so as tohave sufficiently large magneto-resistance.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIG. 1. The rotating shaft 1 is controlled as follows. That is,axial displacement of the rotating shaft 1 is detected by a not-showndisplacement sensor. An output signal from the displacement sensor issupplied to a not-shown magnetic bearing control unit. Control currentsare applied from the control unit to the two electromagnetic coils 10.Thus, magnetic attraction force is generated between the rotary disc 2and the electromagnetic stators 7 so as to bear the rotating shaft 1 ata target position distant from the electromagnetic stators 7 and innon-contact therewith.

Then, as a characteristic portion of the present invention, the deepgroove 16 is provided near the axial center of the rotary disc 2 fixedlyattached to the rotating shaft 1, so as to cover the outer circumferenceof the rotary disc 2. The inner diameter of the deep groove 16 is formedto be smaller than the inner diameter of each of the inside magneticpole teeth 11. That is, the deep groove 16 is located so that the bottomportion thereof is closer to the rotating shaft 1 than the insidemagnetic pole teeth 11 of the electromagnetic stators 7 a and 7 b. Dueto this deep groove 16, an air layer is formed with certain suitablewidth in the axial direction so as to have sufficiently largemagneto-resistance. Accordingly, the one magnetic circuit 13 formed bythe electromagnetic stator 7 a and the rotary disc 2 and the othermagnetic circuit 13 formed by the electromagnetic stator 7 b and therotary disc 2 are insulated from each other magnetically. As a result,formation of an abnormal magnetic circuit designated by the referencenumeral 14 in FIG. 9 can be relieved without increasing the number ofparts, and a magnetic circuit is made independent for each of theelectromagnetic stators 7 a and 7 b. Thus, the electric energy suppliedto the electromagnetic coils 10 can be used effectively for positioncontrol of the rotating shaft 1.

In addition, the two rotary disc pieces 3 are integrally coupled in thebottom portion of the deep groove 16. Therefore, even if maximum stressis generated due to centrifugal force in the rotation-axis-directionpositions of the angular portions 4 where the rotary disc 2 has amaximum outer diameter at the time of high speed rotation, the stress isrelieved so that the fixation between the rotating shaft 1 and therotary disc 2 is always held.

Further, the mass of the rotary disc 2 can be reduced by the formationof the deep groove 16, and the number of fixation parts can be alsoreduced. As a result, the natural frequency of a rotor which might causea problem at the time of high speed rotation can be increased. Thus, itcan be made easy to design the rotor.

Furthermore, the deep groove 16 can be formed easily with a lathe or thelike. Thus, the workability is excellent. Mode 2.

FIG. 2A is a sectional view showing Mode 2 for carrying out the axialmagnetic bearing apparatus according to the present invention, and FIG.2B is a sectional view taken on line X-X in FIG. 2A. Incidentally, inthis Mode 2, the structure of the rotary disc 2 in the above-mentionedMode 1 is modified.

That is, in this Mode 2, as shown in FIGS. 2A and 2B, the rotary disc 2has, on its opposite sides, sleeves 6 to be fixedly attached to therotating shaft 1. Slit-like through holes 17 extending from the outercircumferential portion of the rotary disc 2 to the outercircumferential portion of the rotating shaft 1 are provided at severalplaces in the vicinity of the axial center of the rotary disc 2.Incidentally, these through holes 17 are formed in positions where thethrough holes 17 do not unbalance the rotary disc 2 when the rotary disc2 rotates at a high speed. In addition, an air layer formed by each ofthese fan-shaped slit-like through holes 17 is formed with a certainsuitable diameter to have sufficiently large magneto-resistance. Inaddition, in FIG. 2B, the reference numeral 17 a represents a portion 17a where no slit-like through hole 17 is provided. The portion 17 a playsa role of connection between rotary disc pieces 3 and 3. Incidentally,the other configuration is similar to that in Mode 1, and hencedescription thereof will be omitted.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIGS. 2A and 2B. Means for position control of the rotatingshaft 1 is similar to that in Mode 1.

Then, as a characteristic portion of the present invention, theslit-like through holes 17 are provided in the vicinity of the axialcenter of the rotary disc 2 fixedly attached to the rotating shaft 1,and at several places in the outer diameter portion of the rotary disc2. An air layer formed by these slit-like through holes 17 is formedwith a certain suitable diameter so as to have sufficiently largemagneto-resistance. Accordingly, the magnetic circuit formed by theelectromagnetic stator 7 a and the rotary disc 2 and the magneticcircuit formed by the electromagnetic stator 7 b and the rotary disc 2are relieved from magnetic interference. As a result, formation of anabnormal magnetic circuit designated by the reference numeral 14 in FIG.9 can be relieved without increasing the number of parts, and magneticcircuits formed by the electromagnetic stators 7 a and 7 b are madeindependent of each other. Thus, the electric energy supplied to theelectromagnetic coils 10 can be used effectively for position control ofthe rotating shaft 1.

In addition, the root portion where the rotary disc 2 has a maximumdiameter portion is integrated with the portions 17 a where no slit-likethrough holes 17 are provided. Therefore, even if maximum stress isgenerated due to centrifugal force in the rotation-axis-directionpositions of the angular portions 4 where the rotary disc 2 has amaximum outer diameter at the time of high speed rotation, the stress isrelieved so that the fixation between the rotating shaft 1 and therotary disc 2 is always held.

Further, the mass of the rotary disc 2 can be reduced by the formationof the slit-like through holes 17, and the number of fixation parts canbe reduced. As a result, the natural frequency of a rotor which mightcause a problem when the rotor is rotating at a high speed can beincreased. Thus, design of the rotor can be made easy.

Mode 3.

FIG. 3 is a sectional view showing Mode 3 for carrying out the axialmagnetic bearing apparatus according to the present invention. In thisMode 3, the structure of the rotary disc 2 in the above-mentioned Mode 1is improved partially.

That is, in this mode, electromagnetic stators 7 are located on oppositesides of the rotary disc 2 in each of FIG. 1 and FIGS. 2A and 2B so asto have suitable very small distances from the rotary disc 2 on theopposite sides respectively. The distance between the surface of therotary disc 2 not opposed to any one of an inside magnetic pole tooth 11and an outside magnetic pole tooth 12 of each electromagnetic stator 7and the corresponding surface of the electromagnetic stator 7 opposed tothis surface of the rotary disc 2 is formed to be larger than thedistance between the surface of the rotary disc 2 opposed to the insidemagnetic pole tooth 11 and the outside magnetic pole tooth 12 of theelectromagnetic stator 7 and the corresponding surface of theelectromagnetic stator 7 opposed to this surface of the rotary disc 2.In other words, a ring-like recess portion is formed in the surface ofthe rotary disc 2 not opposed to any one of an inside magnetic poletooth 11 and an outside magnetic pole tooth 12 of each electromagneticstator 7 so that the surface of the rotary disc 2 not opposed to theinside magnetic pole tooth 11 and the outside magnetic pole tooth 12 ofthe electromagnetic stators 7 has a shape depressed deeper than thesurface of the rotary disc 2 opposed to the inside magnetic pole tooth11 and the outside magnetic pole tooth 12 of the electromagnetic stator7. Incidentally, in FIG. 3, the symbol A designates a portion whichshows visually the state that magnetic flux is concentrated between thesurface of the rotary disc 2 opposed to the inside magnetic pole tooth11 of each electromagnetic stators 7 and the inside magnetic pole tooth11 of the electromagnetic stator 7 and between the surface of the rotarydisc 2 opposed to the corresponding outside magnetic pole tooth 12 ofthe electromagnetic stator 7 and the outside magnetic pole tooth 12 ofthe electromagnetic stator 7 due to the above-mentioned structure.Incidentally, the other configuration is similar to that in Mode 1, andhence description thereof will be omitted.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIG. 3. Means for position control of the rotating shaft 1 issimilar to that in Mode 1.

Then, as a characteristic portion of the present invention, theelectromagnetic stators 7 are located on opposite sides of the rotarydisc 2 fixedly attached to the rotating shaft 1 so as to have suitablevery small distances from the rotary disc 2 on the opposite sidesrespectively, and the distance between the surface of the rotary disc 2not opposed to any one of the inside magnetic pole tooth 11 and theoutside magnetic pole tooth 12 of each electromagnetic stators 7 and thesurface of the electromagnetic stator 7 opposed to this surface of therotary disc 2 is formed to be larger than the distance between thesurface of the rotary disc 2 opposed to the inside magnetic pole tooth11 and the outside magnetic pole tooth 12 of the electromagnetic stator7 and the surface of the electromagnetic stator 7 opposed to thissurface of the rotary disc 2. By the difference between these surfacedistances, magnetic flux and leakage flux entering the inside of therotary disc 2 through the surface of the rotary disc 2 not opposed toany one of the inside magnetic pole tooth 11 and the outside magneticpole tooth 12 of each electromagnetic stator 7 are relieved. Thus,magnetic flux generated by each electromagnetic stator 7 can enter theinside of the rotary disc 2 efficiently only through the surface of therotary disc 2 opposed to the inside magnetic pole tooth 11 and theoutside magnetic pole tooth 12 of the electromagnetic stator 7. As aresult, the magnetic flux density in each inside magnetic pole tooth 11and the corresponding outside magnetic pole tooth 12 can be enhancedwithout increasing the number of parts. Thus, the electric energysupplied to the electromagnetic coils 10 can be used more effectivelyfor position control of the rotating shaft 1.

Incidentally, the characteristic configuration of this Mode 3 can bealso combined with the configuration shown in Mode 2.

Mode 4.

FIG. 4A is a sectional view showing Mode 4 for carrying out the axialmagnetic bearing apparatus according to the present invention, and FIG.4B is a sectional view taken on line X-X in FIG. 4A. Incidentally, inthis Mode 4, the structure of the electromagnetic stators 7 in theabove-mentioned Mode 1 is modified partially.

That is, in this mode, slits 18 are provided at several places in theouter circumferential portion of each electromagnetic stator 7 so as tocover the axial length of the electromagnetic stator 7. Incidentally,each of these slits 18 is formed with a certain suitable radial depth sothat an air layer formed by these slits 18 has sufficiently largemagneto-resistance. In addition, the number of the slits 18 and thecircumferential length of each slit 18 are determined suitably so thatcontact portions between each electromagnetic stator 7 and thecorresponding casing 23 are reduced to the utmost, that is, to an extentallowable in attachment strength. Incidentally, the other configurationis similar to that in Mode 1, and hence description thereof will beomitted.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIGS. 4A and 4B. Means for position control of the rotatingshaft 1 is similar to that in Mode 1.

Then, as a characteristic portion of the present invention, the slits 18are provided at several places in the outer circumferential portion ofeach electromagnetic stator 7 disposed with a suitable very smalldistance from the rotary disc 2 on corresponding one of opposite sidesof the rotary disc 2. Each of these slits 18 is formed with a certainsuitable radial depth so that an air layer formed by these slits 18 hassufficiently large magneto-resistance. In addition, the number of theslits 18 and the circumferential length of each slit 18 are determinedsuitably so that contact portions between each electromagnetic stator 7and the corresponding casing 23 are reduced to the utmost, that is, toan extent allowable in attachment strength. Accordingly, generation of amagnetic circuit not contributing to magnetic attraction force formedamong the two electromagnetic stators 7 opposed to each other withrespect to the rotary disc 2, the collar 22 generally made of aniron-based magnetic material, and the casings 23 is relieved. As aresult, the one magnetic circuit formed by the electromagnetic stator 7a and the rotary disc 2 and the other magnetic circuit formed by theelectromagnetic stator 7 b and the rotary disc 2 are insulated from eachother magnetically. Accordingly, formation of an abnormal magneticcircuit designated by the reference numeral 15 in FIG. 10 can berelieved more without increasing the number of parts. Thus, the electricenergy supplied to the electromagnetic coils 10 can be used moreeffectively for position control of the rotating shaft 1.

Incidentally, the characteristic configuration of this Mode 4 can becombined with the configuration shown in Mode 2 or Mode 3.

Mode 5.

FIG. 5 is a sectional view showing Mode 5 for carrying out the axialmagnetic bearing apparatus according to the present invention. In thisMode 5, the structure of the electromagnetic stators 7 in theabove-mentioned Mode 1 is improved partially.

That is, in this mode, ring-like outer circumferential grooves 19 areprovided respectively in the portions of the outer magnetic pole teeth12 of the electromagnetic stators 7 not opposed to the rotary disc 2, soas to extend axially from the side where the rotary disc 2 is located.Each of these outer circumferential grooves 19 is formed with a certainsuitable radial thickness so that an air layer formed by the outercircumferential grooves 19 has sufficiently large magneto-resistance. Inaddition, the axial depth of each outer circumferential groove 19 issecured to an extent allowable in attachment strength, so that theradial magneto-resistance in the outer circumferential groove bottomportion 20 of the corresponding electromagnetic stator 7 can beincreased sufficiently. Incidentally, the other configuration is similarto that in Mode 1, and hence description thereof will be omitted.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIG. 5. Means for position control of the rotating shaft 1 issimilar to that in Mode 1.

Then, as a characteristic portion of the present invention, theelectromagnetic stators 7 are disposed on opposite sides of the rotarydisc 2 so as to have suitable very small distances from the rotary disc2 on the opposite sides respectively, and the outer circumferentialgrooves 19 are provided respectively in the portions of the outermagnetic pole teeth 12 of the electromagnetic stators 7 not opposed tothe rotary disc 2, so as to extend axially from the side where therotary disc 2 is located. Each of these outer circumferential grooves 19is formed with a certain suitable radial thickness so that an air layerformed by these outer circumferential grooves 19 has sufficiently largemagneto-resistance. In addition, the axial depth of each outercircumferential groove 19 is secured to an extent allowable inattachment strength, so that the radial magneto-resistance in the outercircumferential groove bottom portion 20 of the correspondingelectromagnetic stator 7 can be increased sufficiently. Accordingly,generation of a magnetic circuit not contributing to magnetic attractionforce formed among the two electromagnetic stators 7 opposed to eachother with respect to the rotary disc 2, the collar 22 generally made ofan iron-based magnetic material, and the casings 23 is relieved. Inaddition, because the outer circumferential grooves 19 are providedrespectively in the portions of the outside magnetic pole teeth 12 ofthe electromagnetic stators 7 not opposed to the rotary disc 2, themagnetic flux density in the outside magnetic pole tooth 12 of eachelectromagnetic stator 7 is enhanced. As a result, the one magneticcircuit formed by the electromagnetic stator 7 a and the rotary disc 2and the other magnetic circuit formed by the electromagnetic stator 7 band the rotary disc 2 are insulated from each other magnetically.Accordingly, formation of an abnormal magnetic circuit designated by thereference numeral 15 in FIG. 10 can be relieved more without increasingthe number of parts. Thus, the electric energy supplied to theelectromagnetic coils 10 can be used more effectively for positioncontrol of the rotating shaft 1.

Incidentally, the characteristic configuration of this Mode 5 can bealso combined with the configuration shown in Mode 2 or Mode 3. Further,if mechanical stiffness can be kept, the characteristic configuration ofthis Mode 5 can be also combined with the configuration shown in Mode 4.

Mode 6.

FIG. 6A is a sectional view showing Mode 6 for carrying out the axialmagnetic bearing apparatus according to the present invention, and FIG.6B is a sectional view taken on line X-X in FIG. 6A. In this Mode 4, thestructure of the electromagnetic stators 7 in the above-mentioned Mode 1is improved partially. In this Mode 6, the structure of theelectromagnetic stators 7 in the above-mentioned Mode 1 is improvedpartially.

That is, in this mode, the outer diameter of each of the electromagneticstators 7 is formed to be substantially as large as the outer diameterof the rotary disc 2, and rings 21 made of a non-magnetic material areattached to the outer diameter portions of the two electromagneticstators 7. Incidentally, the non-magnetic rings 21 attached between theinner circumferential portions of the casings 23 and the outercircumferential portions of the electromagnetic stators 7 respectivelyare formed with a certain suitable radial thickness so as to havesufficiently large magneto-resistance. Incidentally, the other structureis similar to that in Mode 1, and hence description thereof will beomitted.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIGS. 6A and 6B. Means for position control of the rotatingshaft 1 is similar to that in Mode 1.

Then, as a characteristic portion of the present invention, the outerdiameter of the electromagnetic stator 7 disposed on each of oppositesides of the rotary disc 2 so as to have a suitable very small distancetherefrom is formed to be substantially as large as the outer diameterof the rotary disc 2, and the rings 21 made of a non-magnetic materialare attached to the outer diameter portions of the two electromagneticstators 7. The non-magnetic ring 21 attached between the inner diametersof the casings 23 and the outer diameters of the electromagnetic stators7 are formed with a certain suitable radial thickness so as to havesufficiently large magneto-resistance. Accordingly, generation of amagnetic circuit not contributing to magnetic attraction force formedamong the two electromagnetic stators 7 opposed to each other withrespect to the rotary disc 2, the collar 22 generally made of aniron-based magnetic material, and the casings 23 is prevented. Inaddition, because the outer diameter of each electromagnetic stator 7 isformed to be substantially as large as the outer diameter of the rotarydisc 2, the magnetic flux density in the outside magnetic pole tooth 12of the electromagnetic stator 7 can be enhanced. As a result, the onemagnetic circuit formed by the electromagnetic stator 7 a and the rotarydisc 2 and the other magnetic circuit formed by the electromagneticstator 7 b and the rotary disc 2 are insulated from each othermagnetically perfectly. Accordingly, formation of an abnormal magneticcircuit designated by the reference numeral 15 in FIG. 10 can beprevented more surely. Thus, the electric energy supplied to theelectromagnetic coils 10 can be used more effectively for positioncontrol of the rotating shaft 1.

Incidentally, the characteristic configuration of this Mode 6 can becombined with the configuration shown in any one of Modes 2 to 5.

Mode 7.

FIG. 7 is a sectional view showing Mode 7 for carrying out the axialmagnetic bearing apparatus according to the present invention. In thisMode 7, the collar 22 for relatively positioning the attachment of thetwo electromagnetic stators 7 in the above-mentioned Mode 1 is improved.

That is, in this mode, a collar 22 made of a non-magnetic material isprovided between the two electromagnetic stators 7 disposed on oppositesides of the rotary disc 2 so as to have suitable very small distancesfrom the rotary disc 2 on the opposite sides respectively. Incidentally,the other configuration is similar to that in Mode 1, and hencedescription thereof will be omitted.

By selecting the polarities of electric currents flowing into theelectromagnetic coils 10, for example, two magnetic circuits 13 areformed between the electromagnetic stators 7 and the rotary disc 2 asshown in FIG. 7. Means for position control of the rotating shaft 1 issimilar to that in Mode 1.

Then, as a characteristic portion of the present invention, the collar22 made of a non-magnetic material is provided between the twoelectromagnetic stators 7 disposed on opposite sides of the rotary disc2 so as to have suitable very small distances from the rotary disc 2 onthe opposite sides respectively. Since the collar 22 is formed out of anon-magnetic material, generation of a magnetic circuit not contributingto magnetic attraction force formed between the collar 22 and the twoelectromagnetic stators 7 opposed to each other with respect to therotary disc 2 is prevented. Accordingly, the one magnetic circuit formedby the electromagnetic stator 7 a and the rotary disc 2 and the othermagnetic circuit formed by the electromagnetic stator 7 b and the rotarydisc 2 are insulated form each other magnetically perfectly. Thus,formation of an abnormal magnetic circuit (an abnormal magnetic circuitformed between the electromagnetic stators 7 and the collar 22) can beprevented more surely without any increase in the number of parts. Thus,the electric energy supplied to the electromagnetic coils 10 can be usedmore effectively for position control of the rotating shaft 1. Inaddition, attachment of both the electromagnetic stators 7 can berelatively positioned easily by use of the collar 22 so that theassembly performance is improved.

Incidentally, the characteristic configuration of this Mode 7 can bealso combined with the configuration shown in any one of Mode 2 to Mode6.

In addition, the present invention is not limited to the above-mentionedrespective modes, but includes a wide variety of other modifications.For example, in Node 2, not always by through holes, but by a deepgroove which is deeper than the inner diameter of the inside magneticpole teeth 11 of the electromagnetic stators 7 and which does not reachthe outer circumferential portion of the shaft 1, it is also possible toattain the intended objects.

As described above, according to the structure of axial magnetic bearingapparatus in the present invention, a deep groove or through holes forforming an air layer having large magneto-resistance are provided in thevicinity of the axial center of a rotary disc so as to extend from theouter circumferential portion of the rotary disc toward theabove-mentioned rotating shaft. Accordingly, by the effect of the deepgroove or the through holes, two magnetic circuits formed by respectiveelectromagnetic stators are insulated from each other magnetically. As aresult, formation of an abnormal magnetic circuit can be relieved. Thus,electric energy supplied to electromagnetic coils is effectivelyutilized for position control of the rotating shaft so that the controlperformance can be improved. In addition, the rotary disc is integrallycoupled in the bottom portion of the deep groove or the portion where nothrough hole is provided. Accordingly, even if maximum stress isgenerated due to centrifugal force in the rotation-axis-directionpositions of the inner diameters of the rotary disc having a maximumouter diameter at the time of high speed rotation, the stress isrelieved. Thus, the fixation between the rotating shaft and the rotarydisc is retained surely.

Further, the mass of the rotary disc can be reduced by the formation ofthe deep groove or the through holes, and the number of fixation partscanoe reduced. As a result, the natural frequency of the rotor can beincreased.

Further, according to the present invention, a distance between asurface of the rotary disc located in a position not opposed to any oneof an inside magnetic pole tooth and an outside magnetic pole tooth ofcorresponding one of the electromagnetic stators and a surface of thecorresponding electromagnetic stator opposed to the surface of therotary disc is formed to be larger than a distance between a surface ofthe rotary disc located in a position opposed to each of the insidemagnetic pole tooth and the outside magnetic pole tooth of thecorresponding electromagnetic stator and a surface of the correspondingelectromagnetic stator opposed to the surface of the rotary disc.Accordingly, magnetic flux entering the inside of the rotary discthrough the surface of rotary disc not opposed to any one of the insidemagnetic pole teeth and the outside magnetic teeth of theelectromagnetic stators, and leakage flux escaping to an atmosphere arerelieved, so that the magnetic flux density in the inside magnetic poletooth and the outside magnetic pole tooth of each of the electromagneticstators can be increased. Accordingly, the electric energy supplied tothe electromagnetic coils can be effectively utilized to control theposition of the rotating shaft.

Further, according to the present invention, slits large enough toincrease radial magneto-resistance are provided at several places inouter circumferential portions of the electromagnetic stators.Accordingly, magnetic interference between two magnetic circuits formedby the respective electromagnetic stators is relieved. As a result, itis possible to relieve the formation of an abnormal magnetic circuit.Thus, electric energy supplied to the electromagnetic coils is moreeffectively utilized to control the position of the rotating shaft sothat it is possible to improve the control performance.

Further, according to the present invention, an outer circumferentialgroove for forming an air layer having large magneto-resistance isprovided in a portion of each of the outside magnetic pole teeth of theelectromagnetic stators not opposed to the rotary disc so as to extendaxially from a side where the rotary disc is located. Accordingly,magnetic interference between two magnetic circuits formed by therespective electromagnetic stators is relieved, and the magnetic fluxdensity in the inside magnetic pole tooth and the outside magnetic poletooth of each of the electromagnetic stators can be increased. As aresult, it is possible to relieve the formation of an abnormal magneticcircuit. Thus, electric energy supplied to the electromagnetic coils ismore effectively utilized to control the position of the rotating shaftso that it is possible to improve the control performance.

Further, according to the present invention, an outer diameter of eachof the electromagnetic stators is formed to have substantially as largeas an outer diameter of the rotary disc, and a ring made of anon-magnetic material having a radial thickness enough to form a layerwith large magneto-resistance is interposed between an outercircumferential portion of each of the electromagnetic stators and aninner circumferential portion of corresponding one of the casings towhich the electromagnetic stator is attached. Accordingly, two magneticcircuits formed by the respective electromagnetic stators are insulatedfrom each other magnetically perfectly, and the magnetic flux density inthe inside magnetic pole tooth and the outside magnetic pole tooth ofeach of the electromagnetic stators can be increased. As a result, it ispossible to surely prevent the formation of an abnormal magnetic circuitextending from one electromagnetic stator to the other electromagneticstator through the casings or the collar and extending from the otherelectromagnetic stator to the one electromagnetic stator through therotor disc. Thus, electric energy supplied to the electromagnetic coilsis more effectively utilized to control the position of the rotatingshaft so that it is possible to improve the control performance.

Further, according to the present invention, a collar made of anon-magnetic material for relatively positioning where the pair ofelectromagnetic stators are attached is provided between the pair ofelectromagnetic stators. Accordingly, two magnetic circuits formed bythe respective electromagnetic stators are insulated from each othermagnetically. As a result, it is possible to surely prevent theformation of an abnormal magnetic circuit extending from oneelectromagnetic stator to the other electromagnetic stator through thecollar and extending from the other electromagnetic stator to the oneelectromagnetic stator through the rotor disc. Thus, electric energysupplied to the electromagnetic coils is effectively utilized to controlthe position of the rotating shaft so that it is possible to improve thecontrol performance.

INDUSTRIAL APPLICABILITY

As described above, axial magnetic bearing apparatus according to thepresent invention is suitable for use in bearing of a rotating shaft,particularly a rotating shaft which rotates at a high speed, of rotatingapparatus such as an electric generator, an electric motor, or the like.

1. Axial magnetic bearing apparatus in which a rotary disc made of amagnetic material is fixedly attached to a rotating shaft, while a pairof electromagnetic stators in each of which a ring-like electromagneticcoil for generating magnetomotive force is inserted into a coil slot arefixed to casings respectively so as to be located on opposite sides ofsaid rotary disc with suitable very small distances, and on the basis ofan output signal of a displacement sensor for measuring axialdisplacement of said rotating shaft, magnetic attraction force is madeto act between said rotary disc and each of said electromagnetic statorsso as to bear said rotating shaft in a target position distant from saidelectromagnetic stators and in non-contact therewith, said thrustmagnetic bearing apparatus being characterized in that a distancebetween a surface of said rotary disc existing in a position not opposedto any one of an inside magnetic pole tooth and an outside magnetic poletooth of corresponding one of said electromagnetic stators and a surfaceof said corresponding electromagnetic stator opposed to said surface ofsaid rotary disc is formed to be larger than a distance between asurface of said rotary disc existing in a position opposed to each ofsaid inside magnetic pole tooth and said outside magnetic pole tooth ofsaid corresponding electromagnetic stator and a surface of saidcorresponding electromagnetic stator opposed to said surface of saidrotary disc.